The technique of coating powder particles with another substance for using the powder in various applications is known. With progress in various technical fields, there is a growing desire for a powder having singular properties, in particular, a metal powder or a metal compound powder. Specifically, there is a desire for a powder which combines the properties characteristic of a powder, in particular a metal or metal compound powder, and other properties and hence has multiple functions.
For example, magnetic metal powders as they are cannot be used as raw materials for magnetic color toners because of the color thereof, although their color has not posed any problem in conventional black magnetic toners. Coated powders obtained by the conventional known technique in which a thin metal oxide film is formed on the surface of powder particles to modify the surface for the purpose of, for example, protecting the powder or facilitating the mixing of the powder with synthetic resins or other substances cannot satisfy new requirements in such fields. From this standpoint, it is necessary to provide a powder having a novel constitution not seen in any prior art powder.
The present inventors previously invented a powder having on the surface thereof at least one metal oxide film which has a uniform thickness of from 0.01 to 20 .mu.m and in which the metal contained is different from the metal constituting the base particles, in order to provide a powder, in particular a metal or metal compound powder, which has multiple properties and performs multiple functions so as to satisfy those new requirements (JP-A-6-228604).
If the above powder has two or more layers of the metal oxide film, a special function can be imparted thereto by regulating the thicknesses of the individual layers. For example, when coating films which differ in refractive index are formed on the surface of powder particles each in a thickness corresponding to 1/4 of the wavelength of a light, the coated powder reflects all the light. By applying this technique to a powder comprising magnetic core particles, e.g., a powder of a metal such as iron, cobalt, or nickel or an alloy of such a metal or a powder of iron nitride, it is possible to produce a magnetic powder for magnetic toners which reflects all light and has a bright white color.
A magnetic color toner can be produced from the above powder by forming a colored layer on the powder particles and further forming a resin layer thereon.
The present inventors improved the above powder and invented a powder having a multilayered coating which comprises not metal oxide films alone but metal oxide films stacked alternately with metal films (JP-A-7-90310). This powder has excellent properties which render the powder suitable for use in magnetic color toners, etc.
For producing these powders, it is necessary to form, on powder particles, metal oxide films which each is uniform in thickness. Although this may be attained by precipitating a metal oxide or a metal compound as a precursor therefor from an aqueous solution of a metal salt, this technique is difficult. Hence, the present inventors developed a method which comprises dispersing particles of the above-described powder into a solution of a metal alkoxide and then hydrolyzing the metal alkoxide to form a metal oxide film on the powder particles. By the use of this method, it has become possible to form a metal oxide film which is thin and uniform in thickness, in particular to form a multilayered metal oxide film.
Specifically, the above method is carried out as follows. Powder particles, in particular particles of a metal or metal compound, are dispersed into a solution of a metal alkoxide, and the metal alkoxide is hydrolyzed to obtain a metal oxide on the surface of the particles and deposit a film of the metal oxide on the surface thereof. Subsequently, the metal oxide film is dried. These steps are conducted repeatedly to form a multilayered metal oxide film. By changing the kind of the metal oxide constituting the multilayered metal oxide film, the reflectance for the powder can be changed. It has been found that by selecting a combination of two adjacent metal oxide films so as to result in the maximum reflectance, a powder having a high whiteness can be obtained.
The present inventors thus succeeded in obtaining a powder having a high whiteness by the method described above. In the field of electrophotography and the like, it has become necessary to obtain images having better resolution and high contrast. Accordingly, the magnetic color toners for use in electrophotographic copiers and the like are required not only to have a reduced particle diameter to heighten resolution but also to be colored in a bright hue, in order to form clear images.
Consequently, the powders for use as raw materials for toners are required to have a smaller particle size and a higher whiteness.
In order for a powder to have a higher whiteness, the powder itself should show a higher reflectance. Furthermore, use of a powder having a reduced particle diameter is highly advantageous in that the powder shows enhanced scattering reflection as a whole, is effective in attaining a reduction in toner particle diameter due to the small particle diameter of the powder, and is hence useful for obtaining clear images having heightened resolution.
Forming a multilayered metal oxide film on powder particles in a manner such as that described above serves to increase the diameter of the particles and is hence disadvantageous from the standpoint of obtaining a powder having high whiteness. Although a multilayered metal oxide film in which each layer has a smaller thickness is more advantageous, the method in which a metal alkoxide is used has a problem that it is difficult to obtain a thin metal oxide film having a high density. Furthermore, the prior art technique for obtaining toners of various colors which comprises forming a film consisting mainly of a dye or pigment on white toner particles has a problem that the toners disadvantageously have a large particle diameter due to the film and have poor color brightness.
An object of the present invention is to obtain a powder having a multilayered metal oxide film on the surface thereof, in particular a metal or metal compound powder having any of bright colors including white.
Furthermore, an object of the present invention is to obtain a powder having a multilayered metal oxide film on the surface thereof, in particular a metal or metal compound powder having a reduced particle diameter and any of bright colors including white.
Moreover, an object of the present invention is to provide a magnetic powder suitable for use as a raw material for a magnetic color toner used particularly in electrophotographic copiers and the like, or to provide a thermally conductive powder having electrical insulating properties.
Also, an object of the present invention is to provide a novel process for producing a powder, in particular a metal or metal compound powder, which has any of bright colors including white and a multilayered metal oxide film and which has multiple properties and performs multiple functions.